Piezoelectric polymers are capable of interconverting mechanical and electrical energy, and are therefore candidate materials for biomedical applications such as sensors, actuators, and energy harvesters. In particular, nanowires of these materials are attractive as they can be unclamped, flexible and sensitive to small vibrations. Poly-l-lactic acid (PLLA) nanowires have been investigated for their use in biological applications, but their piezoelectric properties have never been fully characterised, even though macroscopic films and fibres have been shown to exhibit shear piezoelectricity. This piezoelectric mode is particularly interesting for in vivo applications where shear forces are especially relevant, and is similar to what has been observed in natural materials such as bone and DNA. Here, using piezo-response force microscopy (PFM), we report the first direct observation of shear piezoelectricity in highly crystalline and oriented PLLA nanowires grown by a novel template-wetting method. Our results are validated using finite-element simulations and numerical analysis, which importantly and more generally allow for accurate interpretation of PFM signals in soft nanostructured materials. Our work opens up the possibility for the development of biocompatible and sustainable piezoelectric nanogenerators and sensors based on polymer nanowires.

Indium phosphide nanowires were grown by metalorganic molecular beam epitaxy using the selective-area vapor-liquid-solid method. We show experimentally and theoretically that the size of the annular opening around the nanowire has a major impact on nanowire growth rate. In addition, we observed a considerable reduction of the growth rate in dense two-dimensional arrays, in agreement with a calculation of the shadowing of the scattered precursors. Due to the impact of these effects on growth, they should be considered during selective-area vapor-liquid-solid nanowire epitaxy.

Initial or residual stress plays an important role in nanoelectronics. Valley degeneracy in silicon nanowires (SiNWs) is partially lifted due to built-in stresses, and consequently, electron-phonon scattering rate is reduced and device mobility and performance are improved. In this study we use a nonlinear model describing the force-deflection relationship to extract the Young's modulus, the residual stress, and the crystallographic growth orientation of SiNW beams. Measurements were performed on suspended doubly clamped SiNWs subjected to atomic force microscopy (AFM) three-point bending constraints. The nanowires comprised different growth directions and two SiO2 sheath thicknesses, and underwent different rapid thermal annealing processes. Analysis showed that rapid thermal annealing introduces compressive strains into the SiNWs and may result in buckling of the SiNWs. Furthermore, the core-shell model together with the residual stress analysis accurately describe the Young's modulus of oxide covered SiNWs and the crystal orientation of the measured nanowires.

Despite exhibiting weaker piezoelectric properties than commonly used ferroelectric ceramics [ 1 ] (such as barium titanate and lead zirconium titanate) their piezoelectric properties are still technologically viable [ 2 ] while simultaneously possessing a range of advantages over ceramics including being fl exible, low-temperature and solution-processable, light weight, nontoxic and biocompatible, chemically robust, and mechanically stable. [ 3 ] The recent surge in interest has arisen from their suitability in a range of developing technologies such as sensing, [ 4 ] actuation, [ 5 ] nonvolatile memory, [6][7][8] and vibrational energy harvesting applications. [ 2,3 ] For piezoelectric/ pyroelectric applications in particular, there is a requirement for the material to be poled, i.e., to have orientated dipoles, in order for the piezoelectricity/pyroelectricity to manifest. This is typically achieved through externally applied electric fi elds, high temperatures, and/or mechanical stretching.Ferroelectric polymer nanowires grown using a template-wetting method are shown to achieve an orientated "self-poled" structure resulting from the confi ned growth process. Self-poling is highly desirable as it negates the need for high electric fi elds, mechanical stretching, and/ or high temperatures typically associated with poling treatments in ferroelectric polymers, as required for piezoelectric and/or pyroelectric applications. Here, differential scanning calorimetry, infrared spectroscopy, and dielectric permittivity measurements have been presented on as-fabricated template-grown polyvinylidene fl uoride-trifl uoroethylene nanowires, and quantitatively compared with spin-cast fi lms of the same composition that have been electrically poled, both before and after subsequent depoling temperature treatment. The measurements reveal remarkably similar trends between the physical properties of the as-grown nanowires and the electrically poled fi lm samples, providing insight into the material structure of the "self-poled" nano wires. In addition, piezoresponse force microscopy data are presented that allow for unambiguous identifi cation of self-poling in ferroelectric polymer nanostructures. Our results indicate the suitability of the template-wetting approach in fabricating nanowires that can be used directly for piezoelectric/pyroelectric applications, without the need for post-deposition poling/processing.

Quasi-two-dimensional semiconductor materials are desirable for electronic, photonic, and energy conversion applications as well as fundamental science. We report on the synthesis of indium phosphide flag-like nanostructures by epitaxial growth on a nanowire template at 95% yield. The technique is based on in situ catalyst unpinning from the top of the nanowire and its induced migration along the nanowire sidewall. Investigation of the mechanism responsible for catalyst movement shows that its final position is determined by the structural defect density along the nanowire. The crystal structure of the "flagpole" nanowire is epitaxially transferred to the nanoflag. Pure wurtzite InP nanomembranes with just a single stacking fault originating from the defect in the flagpole that pinned the catalyst were obtained. Optical characterization shows efficient highly polarized photoluminescence at room temperature from a single nanoflag with up to 90% degree of linear polarization. Electric field intensity enhancement of the incident light was calculated to be 57, concentrated at the nanoflag tip. The presented growth method is general and thus can be employed for achieving similar nanostructures in other III-V semiconductor material systems with potential applications in active nanophotonics.